It's an oft-repeated maxim that one of the reasons that Windows operating systems are plagued by so many viruses, worms, and security exploits is because they are so popular. Extrapolating on this, many have remarked that if Linux, MacOS, or other OSes become more popular, they will attract the attention of virus writers. That may be true, but the increased attention will not necessarily yield the same quantity of viruses and other exploits, says a Register article. Update: Rebuttal article.

I think that Windows being so open to virii is also a motive. White hats want to highlight problems that are blatantly obvious to all but the ignorant while black hats want to wreak havoc on the net and see Windows as an excellent vehicle.